Grow Mediums How big is your reservoir?

[newtogrow]

I have a 55gallon res. on an ebb and flow system. Im growing 24 plants with it but it is upgradeable to 48 plants.
In a newbie and need a bit of help with PPM can anyone point me in a direction so I can learn. Its getting expensive to totally redo the resevoir every 7 days.

I dont worry about about the ppm I empty and refill every week Id rather spend a little on nutes and know everything is ok hasnt failed me yet 1 litre bottles of plant magic lasted me 3 grows which is $7 a grow which isnt to bad.